The present invention relates to a switched mode power supply (SMPS) control circuit, and to a switched mode power supply (SMPS) comprising such a control circuit.
In an SMPS, the maximum output power is often dependent on the supply voltage. Below a certain input voltage level, the required output power cannot be supplied by the SMPS due to current limitations of the power switches in the SMPS, maximum allowed current in the fuses at the input of the SMPS or maximum allowed current through the transformer. In many cases these limitations will result in audible noise of the transformer.
To prevent the SMPS from starting at a too low input voltage level (and thus preventing audible noise), a circuit has to disable the operation of the SMPS. In prior art systems operating at a rectified line (mains) voltage, external components are required in order to determine the level of the line voltage. These external components require that at least one extra pin is available on the circuit.
In prior art SMPS systems that supply from a rectified line voltage (Vline), detection of the line voltage is performed using e.g. an external voltage divider. Alternatively, a voltage to current converter with an external resistor may be used.
It is a disadvantage of the prior art systems that an extra pin on the integrated circuit (IC) is required in order to determine the level of the line voltage. In addition, the use of external components complicates the implementation of such systems.
Integration of ohmic resistive dividers is very complicated for the following reasons. A high-ohmic resistive divider is not suitable for integration, because high ohmic resistive dividers occupy too much silicon area of the IC. Integration of lower ohmic resistive dividers would lead to too much dissipationxe2x80x94and thereby too much heat generationxe2x80x94and power consumption in the IC.
It is an object of the present invention to provide an improved IC with integrated sensing/monitoring of the line voltage.
The above-mentioned objects are complied with by providing an integrated circuit for providing an enabling signal to a converter, and an SPMS as defined in the independent claims. Advantageous embodiments are defined in the dependent claims.
According to a preferred embodiment, the use of external components for sensing/monitoring the line voltage can be avoided. Furthermore, no extra pin is needed in order to connect the control circuit to external components for sensing the line voltage.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.